1. Field of the Invention
The present invention relates to a method for displaying a defect and an apparatus therefor. Specifically, according to this method and apparatus, a defect, which appears on an object having a solid-color surface to be recognized, such as paper, film, metallic foil or nonwoven fabric cloth, or on an object having a printed surface to be recognized, such as a gravure or offset printed matter, is detected and displayed on the screen of a monitor.
2. Description of the Related Art
Displaying a detected defect as a still image on a monitor is advantageous in finding the type and shape of the defect and realizing an early countermeasure to the defect. According to a conventional method of displaying a defect, a camera for scanning an object to be displayed (or a monitor camera) is situated on the downstream side of a defect detecting apparatus (including a sensor camera for detecting a defect) in a direction in which the object is conveyed. The signal from the sensor camera is shifted in accordance with the object convey speed, and the monitor camera scans a defective area on the object, whereby the defect is displayed as a still image on a monitor.
According to this method, the moving object is illuminated at the position of inspection, and, in this state, the object is scanned over its entire width by the defect detection sensor camera comprising a linear array image sensor. An image signal from the sensor camera is processed to determine the good/bad condition of the object. When the recognized image is determined to be defective, the position of the defect in the width direction of the object is found. Subsequently, the monitor color TV camera, which is movable in the width direction of the object and is disposed on the defect monitor position on the downstream side of the inspection position on which the sensor camera is placed, is moved beforehand to the position where the defect will pass, on the basis of the defect position information. When the moving object passes the defect monitor position, the TV camera scans the defect while this defect is illuminated by a strobe light source. The resultant image signal is displayed on the screen of the monitor as a color still image.
In the above conventional method of displaying a defect, the defect is scanned by the defect detection sensor camera while this defect is illuminated by the inspection illumination device. The position and emission wavelength of the inspection illumination device are controlled to emphasize the defect. On the other hand, the monitor color TV camera scans the object while it is illuminated by the strobe light source which is different from the inspection illumination device. The position and emission wavelength of the strobe light source, however, are not controlled to emphasize the defect. In addition, the resolution of the monitor color TV camera is lower than that of the defect detection sensor camera, Thus, in some cases, the defect in the field of view of the monitor color TV camera is not clearly picked up.
Furthermore, a slight slip occurs in a pulse generator for detecting the speed of movement of the object, and there is a concern that the defect in the field of view of the monitor color TV camera may not be picked up exactly. Consequently, the operator is unable to understand the exact area on the screen of the monitor where the defect is displayed and has some difficulty in finding the defect, in particular, when the object is a printed matter.